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    <link:label id="lab_pwr_RestrictedStockUnitsToBeSettledInCashMember_97E80D29746D3CA7C2FE2177EA944A11_terseLabel_en-US" xlink:label="lab_pwr_RestrictedStockUnitsToBeSettledInCashMember_AB85468428A45C1BB756349CED830EE1" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Restricted Stock Units to be Settled in Cash</link:label>
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    <link:label id="lab_pwr_AmountInCashReceivedForEachRSUIsEqualToFairValueOfThisNumberOfQuantaCommonStockShares_1A063D1B1618AEB9198A2177EA947425_terseLabel_en-US" xlink:label="lab_pwr_AmountInCashReceivedForEachRSUIsEqualToFairValueOfThisNumberOfQuantaCommonStockShares_0C854ED0771956C1A1B4DB18FAB1F28F" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Amount in cash received for each RSU is equal to the fair value of this number of Quanta common stock shares (in shares).</link:label>
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    <link:label id="lab_us-gaap_SeriesGPreferredStockMember_C0EB5DDFB9D922C4B5DDCBD5B038C7F5_terseLabel_en-US" xlink:label="lab_us-gaap_SeriesGPreferredStockMember_C0EB5DDFB9D922C4B5DDCBD5B038C7F5" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Series G Preferred Stock</link:label>
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    <link:label id="lab_us-gaap_OtherAssetsMember_A3F4BF9FAD3B3760484992B444FC898C_label_en-US" xlink:label="lab_us-gaap_OtherAssetsMember_A3F4BF9FAD3B3760484992B444FC898C" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Other Assets [Member]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherAssetsMember" xlink:label="loc_us-gaap_OtherAssetsMember_A3F4BF9FAD3B3760484992B444FC898C" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_OtherAssetsMember_A3F4BF9FAD3B3760484992B444FC898C" xlink:to="lab_us-gaap_OtherAssetsMember_A3F4BF9FAD3B3760484992B444FC898C" xlink:type="arc" />
    <link:label id="lab_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1_verboseLabel_en-US" xlink:label="lab_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:type="resource" xml:lang="en-US">Cash, Cash Equivalents and Restricted Cash [Line Items]</link:label>
    <link:label id="lab_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1_label_en-US" xlink:label="lab_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash and Cash Equivalents [Line Items]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1" xlink:to="lab_us-gaap_CashAndCashEquivalentsLineItems_005ED938EB2D2C290CAB92B444FC7EF1" xlink:type="arc" />
    <link:label id="lab_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35_terseLabel_en-US" xlink:label="lab_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Cash and cash equivalents</link:label>
    <link:label id="lab_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35_label_en-US" xlink:label="lab_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash and Cash Equivalents, at Carrying Value</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35" xlink:to="lab_us-gaap_CashAndCashEquivalentsAtCarryingValue_C2742E34F472327BA50192B444FC1F35" xlink:type="arc" />
    <link:label id="lab_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576_terseLabel_en-US" xlink:label="lab_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Restricted cash and cash equivalents</link:label>
    <link:label id="lab_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576_label_en-US" xlink:label="lab_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Restricted Cash and Cash Equivalents</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalents" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576" xlink:to="lab_us-gaap_RestrictedCashAndCashEquivalents_011FA46503405645E68A92B444FC1576" xlink:type="arc" />
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3_terseLabel_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Total cash, cash equivalents, and restricted cash reported in the statements of cash flows</link:label>
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3_label_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3" xlink:to="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_DA331B085C47B24C22B792B444FDCCC3" xlink:type="arc" />
    <link:label id="lab_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D_terseLabel_en-US" xlink:label="lab_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">ASSETS</link:label>
    <link:label id="lab_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D_label_en-US" xlink:label="lab_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Assets [Abstract]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D" xlink:to="lab_us-gaap_AssetsAbstract_72B54F7DE465A83B28B2EEB2E92B3A5D" xlink:type="arc" />
    <link:label id="lab_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7_terseLabel_en-US" xlink:label="lab_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Current Assets:</link:label>
    <link:label id="lab_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7_label_en-US" xlink:label="lab_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Assets, Current [Abstract]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7" xlink:to="lab_us-gaap_AssetsCurrentAbstract_C6156B0AE9030EF025BEEEB2E92BD4A7" xlink:type="arc" />
    <link:label id="lab_us-gaap_AccountsReceivableNetCurrent_DF4830EDF22FE5DC7EB9EEB2E92D6EAE_terseLabel_en-US" xlink:label="lab_us-gaap_AccountsReceivableNetCurrent_DF4830EDF22FE5DC7EB9EEB2E92D6EAE" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Accounts receivable, net of allowances of $4,757 and $4,465</link:label>
    <link:label id="lab_us-gaap_AccountsReceivableNetCurrent_DF4830EDF22FE5DC7EB9EEB2E92D6EAE_label_en-US" xlink:label="lab_us-gaap_AccountsReceivableNetCurrent_DF4830EDF22FE5DC7EB9EEB2E92D6EAE" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Accounts Receivable, Net, Current</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_DF4830EDF22FE5DC7EB9EEB2E92D6EAE" xlink:type="locator" />
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    <link:label id="lab_us-gaap_ContractWithCustomerAssetNet_B91E21C8697DD7E31149EEB2E92D7EE7_terseLabel_en-US" xlink:label="lab_us-gaap_ContractWithCustomerAssetNet_B91E21C8697DD7E31149EEB2E92D7EE7" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Contract assets</link:label>
    <link:label id="lab_us-gaap_ContractWithCustomerAssetNet_B91E21C8697DD7E31149EEB2E92D7EE7_label_en-US" xlink:label="lab_us-gaap_ContractWithCustomerAssetNet_B91E21C8697DD7E31149EEB2E92D7EE7" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Contract with Customer, Asset, Net</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ContractWithCustomerAssetNet" xlink:label="loc_us-gaap_ContractWithCustomerAssetNet_B91E21C8697DD7E31149EEB2E92D7EE7" xlink:type="locator" />
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    <link:label id="lab_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11_terseLabel_en-US" xlink:label="lab_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Inventories</link:label>
    <link:label id="lab_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11_label_en-US" xlink:label="lab_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Inventory, Net</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InventoryNet" xlink:label="loc_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11" xlink:to="lab_us-gaap_InventoryNet_7ACAFD79B41A1858671AEEB2E92D5A11" xlink:type="arc" />
    <link:label id="lab_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4_terseLabel_en-US" xlink:label="lab_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Prepaid expenses and other current assets</link:label>
    <link:label id="lab_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4_label_en-US" xlink:label="lab_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Prepaid Expense and Other Assets, Current</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4" xlink:to="lab_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_AD6C2F025FB08EC4B02BEEB2E92EDDB4" xlink:type="arc" />
    <link:label id="lab_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53_totalLabel_en-US" xlink:label="lab_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:type="resource" xml:lang="en-US">Total current assets</link:label>
    <link:label id="lab_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53_label_en-US" xlink:label="lab_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Assets, Current</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53" xlink:to="lab_us-gaap_AssetsCurrent_AEF37E79F39055657C18EEB2E92E5C53" xlink:type="arc" />
    <link:label id="lab_us-gaap_PropertyPlantAndEquipmentNet_2B52F6B62C3FB06C32CAEEB2E92F5A77_verboseLabel_en-US" xlink:label="lab_us-gaap_PropertyPlantAndEquipmentNet_2B52F6B62C3FB06C32CAEEB2E92F5A77" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:type="resource" xml:lang="en-US">Property and equipment, net of accumulated depreciation of $1,042,815 and $981,275</link:label>
    <link:label id="lab_us-gaap_PropertyPlantAndEquipmentNet_2B52F6B62C3FB06C32CAEEB2E92F5A77_label_en-US" xlink:label="lab_us-gaap_PropertyPlantAndEquipmentNet_2B52F6B62C3FB06C32CAEEB2E92F5A77" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Property, Plant and Equipment, Net</link:label>
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    <link:label id="lab_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE_terseLabel_en-US" xlink:label="lab_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Other assets, net</link:label>
    <link:label id="lab_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE_label_en-US" xlink:label="lab_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Other Assets, Noncurrent</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE" xlink:to="lab_us-gaap_OtherAssetsNoncurrent_C7FF8BC446CF18A23DA4EEB2E92F9CAE" xlink:type="arc" />
    <link:label id="lab_us-gaap_IntangibleAssetsNetExcludingGoodwill_27D6DB0428FF053DA259EEB2E9306FA0_verboseLabel_en-US" xlink:label="lab_us-gaap_IntangibleAssetsNetExcludingGoodwill_27D6DB0428FF053DA259EEB2E9306FA0" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:type="resource" xml:lang="en-US">Other intangible assets, net of accumulated amortization of $352,520 and $335,507</link:label>
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    <link:label id="lab_us-gaap_LineOfCreditFacilityLineItems_B3C09DB65E7426702C5990F7C1F84C8D_terseLabel_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityLineItems_052C9E062DFB5C69BB074C98E1CF236F" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Line of Credit Facility [Line Items]</link:label>
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    <link:label id="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_4F605D9D8EC5CE6847B390F7C1F8BEA6_terseLabel_en-US" xlink:label="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_13E18F7C5CBA56CCA90ADAAD22C32C79" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Debt instrument basis spread on variable rate</link:label>
    <link:label id="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_4F605D9D8EC5CE6847B390F7C1F8BEA6_label_en-US" xlink:label="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_13E18F7C5CBA56CCA90ADAAD22C32C79" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Basis Spread on Variable Rate</link:label>
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    <link:label id="lab_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_86ACEABE7E82BC4BE47C90F7C1F82B1F_terseLabel_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_FBFE9204E7BB5C3B91BB0944BE37B310" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Commitment fee</link:label>
    <link:label id="lab_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_86ACEABE7E82BC4BE47C90F7C1F82B1F_label_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_FBFE9204E7BB5C3B91BB0944BE37B310" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line of Credit Facility, Unused Capacity, Commitment Fee Percentage</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_FBFE9204E7BB5C3B91BB0944BE37B310" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_FBFE9204E7BB5C3B91BB0944BE37B310" xlink:to="lab_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_FBFE9204E7BB5C3B91BB0944BE37B310" xlink:type="arc" />
    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_CCB18DF32DB05FC276EE90F7C1F85B11_terseLabel_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_4D587AB2F08D54F5A9C2E4392D4EF449" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Reduction in Quanta's funded indebtedness reduced by cash and cash equivalents in excess of this amount</link:label>
    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_CCB18DF32DB05FC276EE90F7C1F85B11_label_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_4D587AB2F08D54F5A9C2E4392D4EF449" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Reduction In Company Maximum Funded Debt And Maximum Senior Debt By All Cash And Cash Equivalents In Excess Of Amount</link:label>
    <link:label id="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_CCB18DF32DB05FC276EE90F7C1F85B11_documentation_en-US" xlink:label="lab_pwr_ReductionInCompanyMaximumFundedDebtAndMaximumSeniorDebtByAllCashAndCashEquivalentsInExcessOfAmount_4D587AB2F08D54F5A9C2E4392D4EF449" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Reduction in Quanta's maximum funded debt and maximum senior debt by all cash and cash equivalents in excess of amount.</link:label>
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    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_29DF99D76E24A3D40B2C90F7C1F8C829_terseLabel_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_441EE8A83B1853B3A452046AB9C85017" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Percentage of capital stock of direct foreign subsidiaries of wholly owned U.S. subsidiaries to secure credit agreement</link:label>
    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_29DF99D76E24A3D40B2C90F7C1F8C829_label_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_441EE8A83B1853B3A452046AB9C85017" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Percentage Of Capital Stock Of Direct Foreign Subsidiaries Required To Secure Credit Agreement</link:label>
    <link:label id="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_29DF99D76E24A3D40B2C90F7C1F8C829_documentation_en-US" xlink:label="lab_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_441EE8A83B1853B3A452046AB9C85017" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Percentage of capital stock of direct foreign subsidiaries required to secure credit agreement.</link:label>
    <link:loc xlink:href="pwr-20180630.xsd#pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement" xlink:label="loc_pwr_PercentageOfCapitalStockOfDirectForeignSubsidiariesRequiredToSecureCreditAgreement_441EE8A83B1853B3A452046AB9C85017" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_9026343BE139986214F090F7C1F9D8E4_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_37A9B6F3A6825FA38E39A23D153CE1BC" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Maximum consolidated leverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_9026343BE139986214F090F7C1F9D8E4_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioMaximum_37A9B6F3A6825FA38E39A23D153CE1BC" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio, Maximum</link:label>
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    <link:loc xlink:href="pwr-20180630.xsd#pwr_DebtInstrumentCovenantLeverageRatioMaximum" xlink:label="loc_pwr_DebtInstrumentCovenantLeverageRatioMaximum_37A9B6F3A6825FA38E39A23D153CE1BC" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_3A916E7DD7B5CE16D02D90F7C1F9CC80_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_910F11DB3A7B545498E163A03040E266" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Acquisition threshold for leverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_3A916E7DD7B5CE16D02D90F7C1F9CC80_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantAcquisitionThresholdForLeverageRatio_910F11DB3A7B545498E163A03040E266" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Acquisition Threshold For Leverage Ratio</link:label>
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    <link:label id="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_86A217DF60B20463B38C90F7C1F93350_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_0E7A1F5303F358ADA532C128EA4A8C02" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Maximum leverage ratio acquisition completed in current and two subsequent quarters</link:label>
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    <link:loc xlink:href="pwr-20180630.xsd#pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum" xlink:label="loc_pwr_DebtInstrumentCovenantLeverageRatioAcquisitionCompletedInCurrentAndTwoSubsequentQuartersMaximum_0E7A1F5303F358ADA532C128EA4A8C02" xlink:type="locator" />
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    <link:label id="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_D83DE58C4EE198ABB05790F7C1F9A3E4_terseLabel_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_5DA9D2E9E1C95858A0C551A43905339E" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Minimum consolidated interest coverage ratio</link:label>
    <link:label id="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_D83DE58C4EE198ABB05790F7C1F9A3E4_label_en-US" xlink:label="lab_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_5DA9D2E9E1C95858A0C551A43905339E" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Interest Coverage Ratio, Minimum</link:label>
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    <link:loc xlink:href="pwr-20180630.xsd#pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum" xlink:label="loc_pwr_DebtInstrumentCovenantInterestCoverageRatioMinimum_5DA9D2E9E1C95858A0C551A43905339E" xlink:type="locator" />
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    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_61BD5643E6855B1336C990F7C1F9FB5D_terseLabel_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_80A61B92C6F65F8E883C529FBD2BC1C1" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Amount of availability under the credit agreement and/or cash and cash equivalents on hand that must be present to allow for cash payments of dividends and stock repurchases</link:label>
    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_61BD5643E6855B1336C990F7C1F9FB5D_label_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_80A61B92C6F65F8E883C529FBD2BC1C1" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Availability Amount To Allow Dividends And Stock Repurchases</link:label>
    <link:label id="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_61BD5643E6855B1336C990F7C1F9FB5D_documentation_en-US" xlink:label="lab_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_80A61B92C6F65F8E883C529FBD2BC1C1" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Amount of availability under the credit agreement and/or cash and cash equivalents on hand that must be present to allow for cash payments of dividends and stock repurchases.</link:label>
    <link:loc xlink:href="pwr-20180630.xsd#pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases" xlink:label="loc_pwr_AvailabilityAmountToAllowDividendsAndStockRepurchases_80A61B92C6F65F8E883C529FBD2BC1C1" xlink:type="locator" />
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    <link:label id="lab_pwr_CrossDefaultProvisions_9273239309B80C9E543C90F7C1F901F6_terseLabel_en-US" xlink:label="lab_pwr_CrossDefaultProvisions_6418C4A0C89E586A8F30484B18A221B8" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Cross default provisions with debt instruments exceeding this amount</link:label>
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    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_71D3B2754D4EE7CCAEB490F7C1AE310F_terseLabel_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_9C0495733DA75E098CEA16F903C9FA65" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Minimum lease payments, 2020</link:label>
    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_71D3B2754D4EE7CCAEB490F7C1AE310F_label_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_9C0495733DA75E098CEA16F903C9FA65" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Operating Leases, Future Minimum Payments, Due in Three Years</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears_9C0495733DA75E098CEA16F903C9FA65" xlink:type="locator" />
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    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_46C8AC19E1EEE2EE46B990F7C1AE2A36_label_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_33F7EE4090A25F478A1A3A41C3BF1BA7" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Operating Leases, Future Minimum Payments, Due in Four Years</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_33F7EE4090A25F478A1A3A41C3BF1BA7" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_33F7EE4090A25F478A1A3A41C3BF1BA7" xlink:to="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears_33F7EE4090A25F478A1A3A41C3BF1BA7" xlink:type="arc" />
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    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears_844A8CD05EEA5EE786EBE295A579ACA3" xlink:type="locator" />
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    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_D979C79392B03DC9CC7D90F7C1AFF78D_terseLabel_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_9A40092949EC51E8B3A237C047AD9667" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Minimum lease payments, Thereafter</link:label>
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    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_9A40092949EC51E8B3A237C047AD9667" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_9A40092949EC51E8B3A237C047AD9667" xlink:to="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter_9A40092949EC51E8B3A237C047AD9667" xlink:type="arc" />
    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_A0C57B69551B1FFDAE0C90F7C1AF3672_totalLabel_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_B9124EF6868C514ABC3AE41038542CDC" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:type="resource" xml:lang="en-US">Total minimum lease payments</link:label>
    <link:label id="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_A0C57B69551B1FFDAE0C90F7C1AF3672_label_en-US" xlink:label="lab_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_B9124EF6868C514ABC3AE41038542CDC" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Operating Leases, Future Minimum Payments Due</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDue_B9124EF6868C514ABC3AE41038542CDC" xlink:type="locator" />
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    <link:label id="lab_pwr_LettersOfCreditAndBankGuaranteesMember_0890126C66BFBFEA9710BB434B2F55EF_terseLabel_en-US" xlink:label="lab_pwr_LettersOfCreditAndBankGuaranteesMember_4FF03FA530D25A658F7A981F81F1C691" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Letters Of Credit and Bank Guarantees</link:label>
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    <link:label id="lab_pwr_LettersOfCreditAndBankGuaranteesMember_0890126C66BFBFEA9710BB434B2F55EF_documentation_en-US" xlink:label="lab_pwr_LettersOfCreditAndBankGuaranteesMember_4FF03FA530D25A658F7A981F81F1C691" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">The Letters Of Credit and Bank Guarantees.</link:label>
    <link:loc xlink:href="pwr-20180630.xsd#pwr_LettersOfCreditAndBankGuaranteesMember" xlink:label="loc_pwr_LettersOfCreditAndBankGuaranteesMember_4FF03FA530D25A658F7A981F81F1C691" xlink:type="locator" />
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    <link:label id="lab_pwr_BorrowingsUnderCreditFacilityMember_5DD5E8499A53D4B86EB1BB434B301918_terseLabel_en-US" xlink:label="lab_pwr_BorrowingsUnderCreditFacilityMember_ADD767DFF2A75728AD3996CBD7824882" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Borrowings Under Credit Facility</link:label>
    <link:label id="lab_pwr_BorrowingsUnderCreditFacilityMember_5DD5E8499A53D4B86EB1BB434B301918_label_en-US" xlink:label="lab_pwr_BorrowingsUnderCreditFacilityMember_ADD767DFF2A75728AD3996CBD7824882" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Borrowings Under Credit Facility [Member]</link:label>
    <link:label id="lab_pwr_BorrowingsUnderCreditFacilityMember_5DD5E8499A53D4B86EB1BB434B301918_documentation_en-US" xlink:label="lab_pwr_BorrowingsUnderCreditFacilityMember_ADD767DFF2A75728AD3996CBD7824882" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">The borrowings under the credit facility.</link:label>
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    <link:label id="lab_pwr_CanadianAndAustralianDollarsMember_15DDD2E0DBBC06815543BB434B312B7D_terseLabel_en-US" xlink:label="lab_pwr_CanadianAndAustralianDollarsMember_305EE90F7E1559F79318CEB7CA2A9BE2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Primarily Canadian and Australian Dollars</link:label>
    <link:label id="lab_pwr_CanadianAndAustralianDollarsMember_15DDD2E0DBBC06815543BB434B312B7D_label_en-US" xlink:label="lab_pwr_CanadianAndAustralianDollarsMember_305EE90F7E1559F79318CEB7CA2A9BE2" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Canadian and Australian dollars [Member]</link:label>
    <link:label id="lab_pwr_CanadianAndAustralianDollarsMember_15DDD2E0DBBC06815543BB434B312B7D_documentation_en-US" xlink:label="lab_pwr_CanadianAndAustralianDollarsMember_305EE90F7E1559F79318CEB7CA2A9BE2" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Canadian and Australian dollars [Member]</link:label>
    <link:loc xlink:href="pwr-20180630.xsd#pwr_CanadianAndAustralianDollarsMember" xlink:label="loc_pwr_CanadianAndAustralianDollarsMember_305EE90F7E1559F79318CEB7CA2A9BE2" xlink:type="locator" />
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    <link:label id="lab_us-gaap_StatementOfCashFlowsAbstract_468A232491330D7BF11DEEB2E892CDC6_label_en-US" xlink:label="lab_us-gaap_StatementOfCashFlowsAbstract_468A232491330D7BF11DEEB2E892CDC6" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Statement of Cash Flows [Abstract]</link:label>
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    <link:label id="lab_us-gaap_GoodwillRollForward_E99201B1E6DE0CA83AAFAAB71408D03A_label_en-US" xlink:label="lab_us-gaap_GoodwillRollForward_E99201B1E6DE0CA83AAFAAB71408D03A" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Goodwill [Roll Forward]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_GoodwillRollForward" xlink:label="loc_us-gaap_GoodwillRollForward_E99201B1E6DE0CA83AAFAAB71408D03A" xlink:type="locator" />
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